Typical miniature electric motors have a metallic cylindrical motor housing formed of mild steel or the like and defining a hollow tubular section and an integral bottom. One end of the motor housing is open to receive a brush base and cover plate, or an integral end cap, that encloses the open end of the motor housing. Within the housing are fixed a pair of opposite permanent magnets, each of which has an arc shape to match the inner wall of the housing. The magnets form between them a volume in the housing for a rotor. The rotor typically includes a cylindrical armature coaxially mounted on a rotor shaft. The rotor shaft extends through the opposite ends of the motor housing. Bearings may be used to rotatably support the rotor shaft in the ends of the housing. For example, the bottom of the motor housing may have an integral flange, into which a bearing can be press-fitted to support one end of the rotor shaft. The end cap may have a similar bearing structure. In this way, the rotor shaft is held in coaxial alignment with the motor housing.
The brush base or end cap supports a pair of brush arms which provide an electrical connection between an external electrical contact of the motor and the armature of the rotor. The brush arms are generally strips of copper having a first end fixed to the periphery of the brush base and an opposite, free end on which is mounted a brush. The brush arms are attached to opposite sides of the brush base. The brushes on the arms face each other at the axis of the motor. A PTC resistor may be incorporated in the end cap in order to protect against over-heating as a result of excessive current flow. When inserted in the housing, the brushes are in slidable contact with a commutator on the rotor shaft. The commutator provides an electrical contact between the wiring of the armature and the brushes. The armature may include any suitable number of wire windings, such as three windings. The external contacts of the brush arms provide direct electrical current (DC) through the brushes and the commutator to the windings in the armature. Electrical current flowing through the armature creates an alternating magnetic field within the housing that interacts with the magnetic field of the permanent magnets. This interaction of magnetic fields creates a force that rotates the rotor. This rotation drives the rotor shaft to provide a mechanical rotational output power source from the rotor. The rotor shaft extends through the bottom of the housing to provide a mechanical power output to drive a gear box or other device. Exemplary miniature electric motors and particular components thereof are shown in, for example, U.S. Pat. Nos. 6,528,922; 6,717,322; 5,294,852; 5,010,264, the disclosures of which are incorporated by reference herein.
Such miniature electric motors can be and are used in variety of applications, including, but not limited to, electrical appliances in the automotive industry and motorized toy vehicles. Miniature motors tend to be a relatively-low cost component of toys and other equipment. Accordingly, it is important that manufacturing costs and complexity for making these motors be minimized. It is also important for various applications incorporating miniature electric motors that the motors do not emit excessive electrical noise (i.e., transient current). The noise (and sound) is caused by sparks generated via commutation, due to, for example, poor contact, sudden change in voltage or back EMF (Electro Magnetic Force). In fact, certain industries, like the automotive industry, have EMC (Electro Magnetic Compatibility) requirements that these motors must satisfy in order to be approved for automotive applications. Such EMC requirements are getting more and more stringent as time goes on.
For example, over the last few decades in the automotive industry, such motors had to meet certain EMC requirements. One method employed for satisfying these EMC requirements in prior art DC motors was to provide a varistor mounted in contact with the commutator to absorb electrical noise from the commutator caused by rotation of the commutator around its terminals. FIGS. 10 and 11 show photographs of two prior art rotors 20 (one by Johnson Electric and one by Microplex NTP Ltd.), including a rotor shaft 22, armature 24, windings 26, commutator 28 and a varistor 30 mounted on the commutator for reducing electrical noise generated by the commutator. The varistor design provided satisfactory results and enabled the motors to meet EMC requirements in the automotive industry prior to 2003.
However, after 2003, the automotive industry established new and stricter EMC requirements that cover a full range EMI—Radiated Emission from 30–890 MHz. Moreover, with respect to EMI—Conducted Interference, the UHF (Ultra-High Frequency Range) was changed to control the radio band FM1 (76–90 MHz) and FM2 (87.5–108 MHz). The prior art designs of FIGS. 10 and 11 incorporating a bare varistor were incapable of meeting these new EMC requirements of the automotive industry. As a result, new methods are needed for satisfying these and similar EMC requirements in, for example, the automotive and any other industry that requires such EMC compliance.
In response to this need, one method that has been used to achieve compliance with the new automotive EMC requirements is shown in FIG. 12. FIG. 12 is photograph of an end cap 36 from an electric motor (by Johnson Electric) that includes a chip capacitor 32 and a choke coil 34 used to suppress the noise generated by the commutator to fulfill the new EMC requirements discussed above. Thus, the Johnson design shown in FIGS. 11 and 12 uses a combination of the varistor 30 on the commutator 28 and a chip capacitor and choke coil in the end cap to achieve the higher level of EMC compliance. More specifically, in the Johnson design, a chip capacitor and choke coil are used to form a “L-C” circuit to suppress the electrical noise generated from the commutation between the two carbon brushes and commutator. However, the choke coil is connected to the carbon brush holder while the chip capacitor is connected between the +ve and −ve terminals. This prior art method provides an indirect method of noise suppression using a simple “L-C” circuit. However, a disadvantage with this prior art technique is that the noise generated will have conducted interference in the brush holder, PTC, terminals and etc. Thus, this method only provides a noise suppression method. Another disadvantage with this prior technique is that it is relatively complex and expensive to produce. Accordingly, a need exists for a more effective and less costly solution to the problem of meeting new EMC requirements for electric motors in the automotive and other industries. The instant invention has been developed to meet this need.
FIG. 13 shows a preferred embodiment of the instant invention. As shown in FIG. 13, the improved rotor design 42 for a DC motor constructed in accordance with the instant invention is similar to the exemplary prior art designs of FIGS. 11 and 12, to the extent that it includes a rotor shaft 22, armature 24, windings 26 and commutator 28. However, in accordance with the instant invention, a ring cap 38 is provided on the commutator for acting a reservoir for absorption of any noise/sound generated from rotation of the commutator around the terminals. In the preferred embodiment, the ring cap includes electric circuits comprised of ceramic chip capacitors soldered on a PCB (Printer Circuit Board), through the use of a Surface Mount Device (SMD) technique. The ring cap is directly connected to the commutator terminals.
The electric circuit of the ring cap 38 may include an L-C (Inductor-Capacitor (or Coil-Capacitor)) circuit (Type A—FIG. 7), an L-C-R circuit (Type B—FIG. 8), or L-C-Varistor circuit (Type C—FIG. 9), depending of the particular application in which the invention is employed. In each of the embodiments, the components are arranged in parallel. An odd number of coils/ceramic chip capacitors are preferably used for DC motor applications. For example, in the embodiment of FIG. 13, three (3) coils/ceramic chip capacitors are used. However, other odd numbers (i.e., 5, 7, . . . ) of coils etc. may also be used depending on the desired result. In terms of cost effectiveness, Type A is the cheapest to produce and Type C is the most expensive. However, in terms of sound reduction effectiveness, Type C is the best and Type B is the second best among the three types. Thus, in accordance with the invention, depending on the extent of noise/sound reduction desired for a given cost (which depends on the size of the reservoir required), one may select the right type of electric circuit for use in the ring cap 38.
While the embodiment of FIG. 13 has particular application to the automotive field where there exist strict EMC standards for controlling noise levels, the invention is not limited to automotive field and can be used in any suitable application. An important advantage of the instant invention is that it provides a direct method for absorption of sound/noise generated from a DC motor. In addition, the production costs can be kept low because the SMD technique can be used when making the ring cap. Further, the chip capacitor and choked coil in the end cap is no longer needed for EMC compliance. As a result, an improved motor design is provided by the instant invention that overcomes several significant disadvantages in the prior art.